1. Field of the Invention
The present invention relates to ultrasonic transducers and, more particularly, to ultrasonic transducer arrays, which are fabricated for low cost, disposable use.
2. Related Art
There are forms of therapy that can be applied within the body of human or other mammalian subject by applying energy to the subject. In hyperthermia, ultrasonic or radio frequency energy is applied from outside of the subject""s body to heat certain body tissues. The applied energy can be focused to a small spot within the body so as to heat a particular tissue or group of tissues to a temperature sufficient to create a desired therapeutic effect. This technique can be used to selectively destroy unwanted tissue within the body. For example, tumors or other unwanted tissues can be destroyed by applying heat to the tissue and raising the temperature thereof to a level (commonly temperatures of about 60xc2x0 C. to 80xc2x0 C.) sufficient to kill the tissue without destroying adjacent, normal tissues. Such a process is commonly referred to as xe2x80x9cthermal ablation.xe2x80x9d Other hyperthermia treatments include selectively heating tissues so as to selectively activate a drug or promote some other physiologic change in a selected portion of the subject""s body. Additional details on the techniques employed in hyperthermia treatments for ablation are disclosed in, for example, copending, commonly assigned PCT International Publication No. WO98/52465, the entire disclosure of which is incorporated herein by reference. Other therapies use the applied energy to destroy foreign objects or deposits within the body as, for example, in ultrasonic lithotripsy.
Often, magnetic resonance imaging devices are utilized in conjunction with ultrasonic treatments so as to ensure that the proper tissues are being affected. Combined magnetic resonance and ultrasonic equipment suitable for these applications are described in greater detail in copending, commonly assigned PCT International Publication No. WO98/52465.
Existing ultrasonic energy emitting devices include piezoelectric resonance units to produce ultrasound waves. The piezoelectric resonance units typically include a substantially rigid frame to which a plurality of separate ultrasound emitting sections are connected and disposed in an array. Each ultrasound emitting section typically includes a rigid backing (such as a block of alumina, glass, or rigid polymer) and a piezoelectric film, such as a polyvinylidene fluoride (PVDF) film, on which a rear electrode is disposed such that the rear electrode is sandwiched between the rigid backing and the piezoelectric film. A front electrode is disposed on an opposite side of the piezoelectric film such that the front electrode faces away from the backing. The electrodes are formed as relatively thin conductive deposits on the surfaces of the piezoelectric film as, for example, by applying an electrically conductive ink on the piezoelectric layer or by sputtering or electroless plating, followed by electroplating.
Each ultrasound emitting section may include numerous pairs of electrodes (one electrode on each side of the piezoelectric film forming a pair) arranged in a further array as, for example, a 3xc3x973 array. Each pair of electrodes and the portion of piezoelectric film disposed between each pair of electrodes forms an independently operable piezoelectric transducer. By applying differential voltage to the two electrodes of the pair, the region of the piezoelectric film between the electrodes can be made to expand and/or contract in a forward-to-rearward direction. Most preferably, the voltage applied to the electrodes is an alternating potential operating at an ultrasonic frequency of about 1-10 MHz, and more commonly 1.0 to 1.8 MHz. This produces a desirable ultrasonic vibration in the piezoelectric film which, in turn, produces ultrasonic waves.
It is desirable to orient the array of ultrasound emitting sections in a relatively curved shape such that a focal length of about 20 cm is obtained. Ultrasonic emitting sections of the curved variety are typically produced by forming a curved structure, often consisting of a plastic frame, and disposing the individual ultrasound emitting sections on the curved structure to produce a unit capable of emitting a focused beam. Unfortunately, this technique is relatively expensive, in part because it requires a substantial number of processing steps to produce and locate the individual ultrasound emitting sections on the curved structure.
Accordingly, there is a need in the art for a new multi-layer piezoelectric transducer structure which may be readily formed into a curved shape to achieve desirable focused ultrasound beam propagation characteristics without undue expense.
A multi-layer piezoelectric transducer according to one aspect of the present invention includes: a first polymeric layer of piezoelectric material having spaced apart first and second surfaces; a second polymeric layer of piezoelectric material having spaced apart first and second surfaces; a first ground layer overlying the first surface of the first polymeric layer; a second ground layer overlying the first surface of the second polymeric layer; and a signal layer sandwiched between the second surfaces of the first and second polymeric layers.
Advantageously, the location of the signal layer sandwiched between the polymeric layers and the ground layers result in: (i) reduced radio frequency (RF) leakage and interference with surrounding electronic circuits; and (ii) higher safety to users, technicians, and/or other foreign bodies that may inadvertently touch the transducer.
The multi-layer piezoelectric transducer may include further polymeric layers having respective ground layers and signal layers in proper interleaving fashion to achieve an overall multi-layer piezoelectric transducer of higher power. Preferably, the polymeric layers are formed from polyvinylidene fluoride (PVDF). Most preferably, the layers of the piezoelectric transducer form a curved shape (e.g., a dome) such that one of the first and second ground layers is located at an ultrasonic emission side of the curve and the other of the first and second ground layers is located at a rear side of the curve. When further polymeric layers are employed, a third, a fourth, a fifth, etc. ground layer may be located at one of the emission and rear sides of the curve.
It is preferred that a piezoelectric transducer is formed into a curved shape by plastically deforming the piezoelectric transducer. The plastic deformation may be obtained, for example, by providing a curve-shaped mold and urging the piezoelectric transducer against the mold such that it deforms into the curved shape. Pressurized gas may be used to push the piezoelectric transducer against a curve-shaped mold. Alternatively, a vacuum may be used to draw the piezoelectric transducer against the curve-shaped mold. Still another method of urging the piezoelectric transducer against a curve-shaped mold may include the steps of providing complimentary curve-shaped mold halves and pressing the piezoelectric transducer between the mold halves to deform the piezoelectric transducer into the curved shape. Preferably, the piezoelectric transducer is fixed into the curved shape using thermoforming techniques (e.g., heating the transducer sufficiently to achieve a plastic deformation state, curving the transducer, and cooling the transducer such that the curved shape is retained).
Advantageously, the piezoelectric transducer is first formed as a substantially flat structure (e.g., using printed circuit forming techniques). Then the transducer is thermally deformed to achieve the curved shape. This results in substantial cost savings and permits previously unattained degrees of freedom in the types of curved shapes which may be obtained. For example, compound curves of the piezoelectric transducer may be achieved using the methods of the present invention.
Preferably, a cured slurry of epoxy, tungsten, and at least one of boron nitride and silicon carbide are disposed at the rear side of the curve to obtain a desirable impedance mis-match at ultrasonic frequencies. Alternatively, one or both of alumina or ceramic material may be disposed at the rear side of the curve as, for example, by spraying or painting molten alumina or ceramic on or near the curve. The impedance mis-match is used to enhance power coming out of the front of the transducer.
The rear layer provides an impedance mis-match and thermal conduction. The piezoelectric layer generates heat during its operation which may be transmitted through the back and front of the transducer.
Other objects, features, and advantages will become apparent to one skilled in the art from the disclosure herein taken in conjunction with the accompanying drawings.